This invention relates to electrophotographic-printing machines and more particularly to a split development system, wherein bias voltages are applied to the development rolls.
Electrophotographic marking is a well-known and commonly used method of copying or printing documents. Electrophotographic marking is performed by exposing a light image representation of a desired document onto a substantially uniformly charged photoreceptive member. In response to exposure by the light image representation, the photoreceptive member discharges so as to create an electrostatic latent image of the desired document on the surface of the photoreceptive member. A development material having toner is then deposited onto the electrostatic latent image so as to form a toner image. That toner image is then transferred from the photoreceptor onto a substrate, such as a sheet of paper. The transferred toner image is then fused to the substrate to form the completed document. Fusing of the toner image to the substrate is typically accomplished by a combination of heat and/or pressure. The surface of the photoreceptive member is then cleaned of residual developing material and recharged in preparation for the production of another document.
Multicolor images by electrophotographic marking can be produced by repeating the above described process once for each color of toner that is employed to form the composite toner image. For example, in one color process referred to herein as the REaD process (recharge expose and develop), a charged photoreceptive surface is exposed to a light image which represents the first component color of a composite color image. The composite color image is produced from a composite toner image of four layers of toner, the first typically being black, followed by magenta, yellow, and cyan. The resulting first electrostatic latent image is then developed with black toner particles to produce the black toner layer for the composite toner image. The charge, expose and develop process is then repeated to form a toner layer of the second component color of the composite color image. In an image-on-image process color (IOI), the subsequent component toner layers may overlay the previous component toner layers to thereby form a full spectrum of colors by their interaction. Alternately, the process can involve image-next-to-image (INI) wherein the component toner layers are positioned adjacent each other. Image-next-to-image is typically employed, for example, in highlight color printing. The INI process, typically has two color toners wherein one component color is the highlight on the document. However, the INI process can clearly be implemented with additional color toners. In the IOI process, the color toner particles of the component toner layers are placed in a superimposed registration so that the desired composite color images are formed. The composite toner image in either the REaD 101 process or REaD INI process is then transferred from the photoreceptive member and onto the substrate.
The REaD IOI and REaD INI processes can be implemented in a variety of configurations of an electrophotographic printing machine. In a single pass printing arrangement, the final composite toner image is produced in a single pass of the photoreceptive member through a printing machine having multiple charging development and exposure stations. Typically, four charging stations and four exposure stations are implemented to recharge, expose and develop each component color toner layer of the desired final four color composite toner image. Alternately in a multiple pass arrangement, the photoreceptive member cycles past a single charging station, a single exposure station, and multiple development stations. The photoreceptive member typically will cycle four times, one cycle for each component toner layer. In either configuration of an electrophotographic-printing machine, the composite toner image its subsequently transferred from the photoreceptive member to the substrate in a single step. The transfer can be directly to the substrate or via an intermediate toner support member such as a belt or drum.
Background print quality defects can be a serious problem with any color print architecture. One such defect is a halo effect, in which a white edge surrounds a toned area. The halo effect occurs when fringe fields, caused by surface potential differences, are generated at the edge of a toned area. The fringe field pulls charged toner particles away from the edges of the area to receive toner, resulting in a white untoned edge surrounding a toned area. In the case of a monochrome process, the white edge surrounding the toned area is not visible, because the background area is white. In the case of a multicolor process, with one color area surrounded by another color, the white edge is visible.
The following disclosures may be relevant and/or helpful in providing an understanding of some aspect of the present invention:
U.S. Pat. No. 5,032,872 to Folkins et al. discloses a developer apparatus including a reservoir for storing a supply of developer material, and a magnetic brush roll for transporting material from the reservoir to each of two donor rolls in a single housing.
U.S. Pat. No. 4,266,868 to Bresina et al. discloses a development apparatus wherein a magnetic brush roller delivers a single component developer directly from a reservoir to a photoconductive surface and also transfers the developer from the reservoir to a second magnetic brush roller.